1.55 - Non-centered parameterizations

- added dmvnormNC density for map2stan, automating non-centered
adaptive priors for multilevel models, using Cholesky factors
internally. This can substantially speed up sampling for many
multilevel models. See example in README.
- added a dashboard() function for checking all diagnostics on a
map2stan or Stan model fit
- added a lognormal template for map2stan: dlnorm in R, lognormal in
Stan
- trace plot grids now default to 3 columns